Method for multi layer enameling and coating compounds for said method

ABSTRACT

A process for lacquer coating substrates with a colored and/or effect base lacquer and a clear lacquer topcoat, in which a colored and/or effect base lacquer layer of a base lacquer coating composition is applied onto a substrate and is provided in a wet-on-wet process with a clear lacquer coating. Before being jointly stoved or jointly cured with the base lacquer layer, the uncured clear lacquer layer is exposed to high-energy radiation. In the clear lacquer coating composition, the resin solids content contains 50 to 98 wt. % of a system A) thermally curable by addition and/or condensation reactions, which system contains substantially no free-radically polymerisable double bonds and substantially no groups capable of reacting in another way with free-radically polymerisable double bonds of a system B). The resin solids content further contains 2 to 50 wt. % of a system B) which is curable under the action of high-energy radiation by free-radical polymerisation of olefinic double bonds, wherein the weight percentages adds up to 100 wt. % and the C═C equivalent weight of the total resin solids content of A) and B) is between 300 and 10000. Clear lacquer coating compositions made by the process are also described.

FIELD OF THE INVENTION

This invention relates to a process for lacquer coating substrates witha coloured and/or effect base lacquer and a clear lacquer topcoat and toclear lacquer coating compositions usable in this process.

BACKGROUND OF THE INVENTION

Modern automotive original lacquer coatings generally consist of a baselacquer/clear lacquer topcoat, which is applied onto a body which hasbeen electrophoretically primed and coated with surfacer. In suchcoating processes, the coloured and/or effect base lacquer and the clearlacquer are preferably applied wet-on-wet, i.e. the base lacquer andsubsequently applied clear lacquer are jointly stoved.

The optical/aesthetic qualities of such base lacquer/clear lacquer twolayer lacquer coatings are substantially influenced by the quality ofthe clear lacquer layer. ideally, the texture of the clear lacquer layeris identical on the horizontal and vertical surfaces of athree-dimensional substrate, for example an automotive body, which maybe ascertained, for example, by determining the long- and short-wavecomponents of the surface texture of the clear lacquer surface. Thisideal situation is, however, not straightforward to achieve in practice.For example, differences in surface texture or leveling of the outerclear lacquer layer may occur as, once applied and in particular duringthe heating phase of the stoving process, the clear lacquers have atendency to run or sag on non-horizontal surfaces due to atemperature-determined reduction in viscosity. There are various ways ofcounteracting unwanted running phenomena in clear lacquers. For example,additives having a favourable effect on running behaviour are added tothe stoving clear lacquers.

However, this frequently results in reduced gloss of the clear lacquerlayer, if the clear lacquer layer is compared with a clear lacquer layerproduced from a corresponding, additive-free stoving clear lacquer.Another possibility for counteracting unwanted running phenomena inclear lacquers involves rotating the substrate around an axis afterapplication of the stoving lacquer during the flashing-off and/orstoving operations, an approach which is disclosed, for example, inEP-B-0 261 644.

Longer term retention of the optical/aesthetic qualities of baselacquer/clear lacquer two coat lacquer coatings, in particular inautomotive applications, is substantially determined by the clearlacquer layer's resistance to chemicals and scratching, which is inparticular manifested over a relatively long period of service by agreater or lesser resistance to acid rain and washing scratches.

EP-A-0 000 407 describes a radiation-curing coating composition based ona hydroxy-functional polyester resin esterified with acrylic acid, avinyl compound, a photoinitiator and a polyisocyanate. In a first curingstage, the coating is radiation-cured by UV light and in a second curingstage hydroxyl/isocyanate crosslinking imparts the final hardness to thecoating composition. The second curing stage may proceed at elevatedtemperature.

EP-A-0 247 563 describes a radiation-curable coating composition basedon a poly(meth)acryloyl-functional compound, a polyolmono(meth)acrylate, a polyisocyanate, a light stabiliser and aphotoinitiator. Radiation curing proceeds by UV light. The hydroxylfunctions in one part of the radiation-curing binder provide anadditional possibility for curing the polyisocyanate, for example inshaded areas.

EP-A-0 540 884 describes a process for the production of a multilayerlacquer coating by applying a clear lacquer coating onto a dried orcrosslinked coloured and/or effect base lacquer layer, wherein the clearlacquer coating is applied from a coating composition which is curablesolely by free-radical and/or cationic polymerisation and curing isinitiated and/or performed by high-energy radiation.

SUMMARY OF THE INVENTION

The object of the invention is to provide a process which may beperformed wet-on-wet for lacquer coating substrates with a colouredand/or effect base lacquer and a clear lacquer topcoat with outstandingoptical/aesthetic qualities. The process to be provided should preventrunning of the clear lacquer on vertical surfaces and clear lacquerlayers should be formed which have reduced susceptibility to chemicalsand scratching, in particular to acid and washing scratches. Inparticular, it should be possible to perform the process using clearlacquer coating compositions of a simple composition, preferably suchcompositions which may be formulated from conventional, for examplecommercially available components, i.e. any constituents of the clearlacquer coating composition which are not commercially available shouldbe simple to synthesis.

It has been found that this object may surprisingly be achieved by aprocess for the production of a multilayer lacquer coating, in which acoloured and/or effect base lacquer layer of a base lacquer coatingcomposition is applied onto a substrate and is provided in a wet-on-wetprocess with the clear lacquer coating, wherein, before being jointlystoved or jointly cured with the base lacquer layer, the uncured clearlacquer layer is exposed to high-energy radiation, characterised in thatthe clear lacquer coating composition used is one, the resin solidscontent of which consists of 50 to 98 wt. % of a system A) thermallycurable by addition and/or condensation reactions, which system containssubstantially no free-radically polymerisable double bonds andsubstantially no groups capable of reacting in another way withfree-radically polymerisable double bonds of system B), and of 2 to 50wt. % of a system B) which is curable under the action of high-energyradiation by free-radical polymerisation of olefinic double bonds,wherein the weight percentages add up to 100 wt. % and wherein the C═Cequivalent weight of the total resin solids content from A) and B) isbetween 300 and 10000.

The weight percentages here and below relate in each case to thecomposition of the resin solids content (=total of all film-formingconstituents of the clear lacquer coating composition=sum of allconstituents of systems A) and B)). Resin solids content here and belowmeans the theoretical resin solids content, which takes account neitherof any evaporative losses nor of stoving losses during application andcuring of the clear lacquer coating composition. This resin solidscontent, together with any further non-volatile constituents optionallypresent in the clear lacquer coating composition, constitutes the solidscontent thereof.

The phrase “contains substantially no free-radically polymerisabledouble bonds and substantially no groups capable of reacting withfree-radically polymerisable double bonds of system B)” means here andbelow that, apart from a possible content arising from industrialcontamination, system A) contains no free-radically polymerisable doublebonds nor any groups capable of reacting in another way withfree-radically polymerisable double bonds of system B).

This invention furthermore relates to the clear lacquer coatingcompositions usable in the process according to the invention, which arecharacterised in that the resin solids content thereof consists of 50 to98 wt. % of a system A) thermally curable by addition and/orcondensation reactions, which system contains substantially no freeradically polymerisable double bonds and substantially no groups capableof reacting in another way with free radically polymerisable doublebonds of system B), and of 2 to 50 wt. % of a system B) which is curableunder the action of high-energy radiation by free-radical polymerisationof olefinic double bonds, wherein the weight percentages add up to 100wt. % and wherein the C═C equivalent weight of the total resin solidscontent from A) and B) is between 300 and 10000.

The coloured and/or effect base lacquer coating layer may be produced inthe process according to the invention using per se known colouredand/or effect base lacquer coating compositions applicable by spraying,as are used for the production of base lacquer/clear lacquer two layerlacquer coatings and large numbers of which are known, for example, fromthe patent literature.

Base lacquers usable for the production of the coloured and/or effectbase lacquer layer in the process according to the invention may bephysically drying or crosslinkable with the formation of covalent bonds.Base lacquers which crosslink with the formation of covalent bonds maybe intrinsically or extrinsically crosslinking systems.

The coloured and/or effect base lacquers usable in the process accordingto the invention are liquid coating compositions. They may comprisesingle or multi-component coating compositions, with single componentcompositions being preferred. They may comprise systems based on organicsolvents or they preferably comprise aqueous base lacquers, the bindersystems of which are suitably, for example anionically, cationically ornonionically, stabilised.

The base lacquer coating compositions usable in the process according tothe invention are conventional lacquer systems which contain one or moreconventional film-forming binders. In the event that the binders are notintrinsically crosslinking or self-drying, they may optionally alsocontain crosslinking agents. No restrictions apply to either the bindersor crosslinking agents. Examples of usable film-forming binders arepolyester, polyurethane and/or (meth)acrylic copolymer resins.(Meth)acrylic here and below means acrylic and/or methacrylic. Thepreferred aqueous base lacquers preferably contain polyurethane resins,particularly preferably in a minimum proportion of 15 wt. %, relative tothe solid resin content of the aqueous base lacquer. Selection of the isoptionally present crosslinking agents is non-critical and is determinedin accordance with the functionality of the binders, i.e. thecrosslinking agents are selected such that they have a reactivefunctionality complementary to the functionality of the binders.Examples of such complementary functionalities between binder andcrosslinking agent are: hydroxyl/methylol ether, hydroxyl/freeisocyanate, hydroxyl/blocked isocyanate, carboxyl/epoxide. Provided thatthey are mutually compatible, two or more such complementaryfunctionalities may simultaneously be present in a base lacquer. Thecrosslinking agents optionally used in the base lacquers may be presentindividually or as a mixture.

Apart from the conventional physically drying and/or chemicallycrosslinking binders, the base lacquers used in the process according tothe invention also contain inorganic and/or organic colouring pigmentsand/or effect pigments, such as for example titanium dioxide, iron oxidepigments, carbon black, azo pigments, phthalocyanine pigments,quinacridone pigments, metal pigments, for example made from titanium,aluminium or copper, interference pigments, such as for examplealuminium coated with titanium dioxide, coated mica, graphite effectpigments, lamellar iron oxide, lamellar copper phthalocyanine pigments.

The base lacquers may furthermore contain conventional lacqueradditives, such as for example extenders, catalysts, leveling agents,anticratering agents or in particular light stabilisers, optionallycombined with antioxidants.

The coloured and/or effect base lacquer layer applied by spraying may beapplied as a two layer base lacquer system consisting of a first,modified base lacquer layer and a second base lacquer layer whichdetermines the actual colour tone of the multilayer lacquer coating. Inthis case, the first base lacquer is modified, for example by containingadditional modifying binder and/or extender components. It may, forexample, be produced from the subsequently applied base lacquer whichdetermines the actual colour tone by mixing with the additionalcomponents.

The clear lacquer coating compositions usable in the process accordingto the invention are liquid clear lacquers. They may comprise singleand/or multicomponent clear lacquer coating compositions, with singlecomponent, storage-stable clear lacquer coating compositions beingpreferred. They may be solvent-free or water-borne, preferably aqueousemulsions, or they preferably comprise solvent-based systems. It isessential to the invention that the clear lacquer coating compositionshave a resin solids content which consists of 50 to 98 wt. %, preferablyof 70 to 95 wt. %, of a system A) thermally curable by addition and/orcondensation reactions, which system contains substantially nofree-radically polymerisable double bonds and substantially no groupscapable of reacting in another way with free-radically polymerisabledouble bonds of system B), and of 2 to 50 wt. %, preferably of 5 to 30wt. %, of a system B) which is curable under the action of high-energyradiation by free-radical polymerisation of olefinic double bonds,wherein the weight percentages add up to 100 wt. % and wherein the C═Cequivalent weight of the entire resin solids content formed from thetotal of A) and B) is between 300 and 10000, preferably between 350 and3000, in particular between 400 and 1500.

The systems A) thermally curable by addition and/or condensationreactions, which systems contain substantially no free-radicallypolymerisable double bonds and substantially no groups capable ofreacting in another way with free-radically polymerisable double bondsof system B), comprise binders into which optionally reactive diluents(compounds which are chemically incorporated into lacquer film duringstoving) and optionally crosslinking systems for clear lacquer coatingcompositions are incorporated, preferably for such clear lacquer coatingcompositions as may be used in the production of base lacquer/clearlacquer two layer lacquer coatings. The binders particularly preferablycomprise conventional binders known to the person skilled in the art,into which optionally reactive diluents and optionally crosslinkingsystems for clear lacquer coating compositions are incorporated, as areprior art in the production of base lacquer/clear lacquer two layercoatings and are known, for example, from numerous patent documents.

The systems A) thermally curable by addition and/or condensationreactions consist of one or more film-forming binder, optionally of oneor more reactive diluents, and, if the binders are not intrinsicallycrosslinking, of one or more crosslinking agents for the binders andoptionally present reactive diluents. Preferred extrinsicallycrosslinking systems A) are those in which binders, optionally presentreactive diluents and crosslinking agents are present in astoichiometric ratio adapted to the desired degree of crosslinking ofgenerally 60 to 90 parts of binder and optionally reactive diluents to40 to 10 parts of crosslinking agent, wherein the total amounts to 100parts. No restrictions in principle apply to the binder, the reactivediluents and the crosslinking agents, except that they containsubstantially no free-radically polymerisable double bonds andsubstantially no groups capable of reacting in another way withfree-radically polymerisable double bonds of system B) and may bethermally inducibly cured by addition and/or condensation reactions.Film-forming binders which may be used are, for example, polyester,polyurethane, (meth)acrylic copolymer and/or triazine-derived resins.The binders preferably contain substantially no aromatic structuralunits and are based on aliphatic and/or cycloaliphatic units. The(meth)acrylic copolymer resins may contain aromatic structural units,for example in the form of copolymerised styrene. In the case ofintrinsically crosslinking binders, these contain mutually complementaryreactive groups as the basis for covalent crosslinking. In the case ofthe preferred extrinsically crosslinking binders, the per senon-critical selection of the crosslinking agents present in this caseis determined by the functionality of the binders, i.e. the crosslinkingagents are selected such that they have a reactive functionalitycomplementary to the functionality of the binders, wherein thefunctional groups may react together thermally by addition and/orcondensation. Examples of addition reactions suitable for crosslinkingsystems A) are ring-opening addition of an epoxide group onto a carboxylgroup forming an ester and a hydroxyl group, the addition of a hydroxylgroup onto an isocyanate group forming a urethane group, while examplesof condensation reactions suitable for crosslinking systems A) are thereaction of a hydroxyl group with a blocked isocyanate group forming aurethane group and eliminating the blocking agent, the reaction of ahydroxyl group with an N-methylol group eliminating water, the reactionof a hydroxyl group with an N-methylol ether group eliminating theetherification alcohol, the transesterification reaction of a hydroxylgroup with an ester group eliminating the esterification alcohol, thetransurethanisation reaction of a hydroxyl group with a carbamate groupeliminating alcohol, the reaction of a carbamate group with anN-methylol ether group eliminating the etherification alcohol. Providingthat they are mutually compatible, two or more complementaryfunctionalities may simultaneously be present in a system A) thermallycurable by addition and/or condensation reaction, such that two or moreof the various types of reaction stated above by way of example mayoccur during stoving. The crosslinking agents optionally used in thesystems A) may be present individually or as a mixture.

Preferred single-component systems A) are those which havehydroxy-functional (meth)acrylic copolymers, polyester resins and/orpolyurethane resins individually or as a mixture as the binder andoptionally additionally contain hydroxy-functional reactive diluents,while the crosslinking agents for the hydroxy-functional binders andoptionally present hydroxy-functional reactive diluents comprise, forexample, triazine-based components, for example amino resins, inparticular melamine resins, which crosslink with the hydroxyl groupsthereof forming ether and/or ester groups and/or transesterificationcrosslinking agents and/or blocked polyisocyanates.

Preferred two-component systems A) are those which havehydroxy-functional (meth)acrylic copolymers, polyester resins and/orpolyurethane resins individually or as a mixture as the binder andoptionally additionally contain hydroxy-functional reactive diluents,while the crosslinking agents for the hydroxy-functional hinders andoptionally present hydroxy-functional reactive diluents comprise freepolyisocyanates.

Examples of hydroxy-functional polyester resins usable as the binder inthe systems A) are conventional, for example those having a numberaverage molecular weight (Mn) of 500 to 5000, preferably of 1000 to 3000and hydroxyl values of 30 to 450, preferably of 50 to 280 mg of KOH/g.

Examples of hydroxy-functional polyurethane resins usable as the binderin the systems A) are conventional, for example those having a numberaverage molecular weight (Mn) of 500 to 5000, preferably of 1000 to 3000and hydroxyl values of 30 to 450, preferably of 50 to 280 mg of KOH/g.

Examples of hydroxy-functional (meth)acrylic copolymers usable as thebinder in the systems A) are conventional, for example those having anumber average molecular weight (Mn) of between 1000 and 10000 andhydroxyl values of 30 to 300, preferably of 50 to 250 mg of KOH/g. The(meth)acrylic copolymers may have been produced in the presence ofoligomeric or polymeric polyester and/or polyurethane resins.

Examples of hydroxy-functional reactive diluents usable in the systemsA) are low molecular weight compounds having at least two hydroxylgroups per molecule and hydroxyl values in the range from 250 to 700 mgof KOH/g. Oligomeric or polymeric polyols, such as polyether polyols,oligoester polyols, polycarbonate polyols and oligourethane polyols aresuitable. Commercial products are, for example, suitable, such aspolycaprolactone polyols obtainable by reacting polyols withcaprolactone, polyether polyols, such as for example triethylene glycol,obtainable by reacting oxirane compounds with polyols and/or water, oroligourethane polyols obtainable by reacting polyamines with cycliccarbonates.

Examples of polyisocyanates usable in free or blocked form in thesystems A) as crosslinking agents for the hydroxy-functional binders andoptional reactive diluents are cycloaliphatic or aliphatic diisocyanatessuch as tetramethylene diisocyanate, 1,6-hexamethylene diisocyanate,1,12-dodecane diisocyanate, 1,3- and 1,4-cyclohexane diisocyanate,isophorone diisocyanate, biscyclohexylmethane diisocyanate or mixturesthereof.

Other suitable isocyanates apart from these simple isocyanates are thosewhich contain heteroatoms in the residue linking the isocyanate groups.Examples of these are polyisocyanates having carbodiimide groups,allophanate groups, isocyanurate groups, uretidione groups, urethanegroups, acylated urea groups and biuret groups.

Known polyisocyanates which are primarily used in the production oflacquers are particularly suitable, for example modification products ofthe above-stated simple polyisocyanates containing biuret, isocyanurateor urethane groups, in particular tris-(6-isocyanatohexyl)biuret, theisocyanurate derived from isophorone diisocyanate or hexane diisocyanateor polyisocyanates having low molecular weight urethane groups, as maybe obtained by reacting excess isophorone diisocyanates with simplepolyhydric alcohols of the molecular weight range from 62 to 300, inparticular with trimethylolpropane. Any desired mixtures of the statedpolyisocyanates may, of course, also be used.

Suitable polyisocyanates furthermore include known prepolymers havingterminal isocyanate groups, as are in particular obtainable by reactingthe above-stated simple polyisocyanates, especially diisocyanates, withsub-stoichiometric quantities of organic compounds having at least twogroups capable of reacting with isocyanate groups. Suitable blockingagents for the polyisocyanate crosslinking agents described above arethe conventional compounds, such as conventional CH-acidic, NH—, SH— orOH— functional compounds. These may be used for blocking individually oras a mixture. Examples are CH-acidic compounds such as acetylacetone orCH-acidic esters, such as for example acetoacetic acid alkyl esters,malonic acid dialkyl esters, aliphatic or cycloaliphatic alcohols, suchas n-butanol, isopropanol, tert.-butanol, furfurol, 2-ethylhexanol,cyclohexanol; oximes such as methyl ethyl ketoxime, acetone oxime,cyclohexanone oxime, acetophenone oxime, lactams such as ε-caprolactamor 2-pyrrolidone, imidazoles such as 2-methylimidazole, pyrazoles suchas 2,3-dimethylpyrazole. CH-acidic esters and/or oximes are preferredblocking agents.

Examples of triazine-based components which crosslink with thehydroxy-functional binders and optionally additionally present reactivediluents forming ether groups are, for example, amino resinsconventional as crosslinking agents, preferably melamine resins, such asfor example methanol-etherified melamine resins, such as the commercialproducts Cymel 325, Cymel 327, Cymel 350 and Cymel 370, Maprenal MF 927.Further examples of usable melamine resins are butanol- orisobutanol-etherified melamine resins such as for example the commercialproducts Setamin US 138 or Maprenal MF 610; co-etherified melamineresins which are both butanol- and methanol-etherified, such as forexample Cymel 254, as well as hexamethoxymethylmelamine (HMMM), such asfor example Cymel 301 or Cymel 303, wherein the latter may require anexternal acid catalyst, such as for example p-toluenesulfonic acid forcrosslinking. Such acid catalysts may optionally be ionically ornonionically blocked with amines or polyepoxides.

Transesterification crosslinking agents, such as for example preferablytris(alkoxycarbonylamino)triazine, are examples of triazine-basedcomponents in the systems A) which crosslink with the hydroxy-functionalbinders and optionally additionally present reactive diluents to formester groups, in particular to form urethane groups (carbamic acid estergroups).

It is particularly preferred in the process according to the inventionto use clear lacquer coating compositions, the resin solids content ofwhich as system A) contains an extrinsically crosslinking systemcomprising a hydroxy-functional binder, in particular based on(meth)acrylic copolymer, and one or more amino resins as thecrosslinking agent.

The systems B) comprise free-radically curing systems, i.e.free-radicals are formed by the action of high-energy radiation on thesystem B), which free radicals initiate crosslinking of the system B) byfree-radical polymerisation of the olefinic double bonds thereof. Thesystems B) are of a composition such that, under the application andcuring conditions of the clear lacquer coating composition containingthem, they do not react or do not react to an appreciable extent withthe particular system A) present in said clear lacquer coatingcomposition.

The free-radically curing systems comprise prepolymers, such as polymersor oligomers, which have free-radically polymerisable, olefinic doublebonds, in particular (meth)acryloyl groups, in the molecule. Apart fromthe free-radically polymerisable double bonds, the prepolymerspreferably contain no functional groups which can bring aboutcrosslinking with constituents of system A). The prepolymers may bepresent in combination with reactive diluents, i.e. reactive liquidmonomers. The free-radically curing systems preferably also containphotoinitiators.

Examples of prepolymers or oligomers are (meth)acrylic-functional(meth)acrylic copolymers, which may contain aromatic structural units,for example in the form of copolymerised styrene, as well as epoxy resin(meth)acrylates, polyester (meth)acrylates, polyether (meth)acrylates,polyurethane (meth)acrylates, unsaturated polyesters, unsaturatedpolyurethanes or silicone (meth)acrylates preferably containing noaromatic structural units, based on aliphatic and/or cycloaliphaticunits and having number average molecular weights (Mn) preferably in therange from 200 to 10000, particularly preferably from 500 to 3000 andhaving on average 2 to 20, preferably 3 to 10 free-radicallypolymerisable, olefinic double bonds per molecule.

If reactive diluents are used, they are preferably used in quantities of1 to 50 wt. %, preferably of 5 to 30 wt. %, relative to the total weightof prepolymers and reactive diluents. These comprise defined lowmolecular weight compounds, which may be mono-, di- or polyunsaturated.Examples of such reactive diluents are: (meth)acrylic acid and theesters thereof, maleic acid and the semi-esters thereof, vinyl acetate,vinyl ethers, substituted vinyl-ureas, ethylene and propylene glycoldi(meth)acrylate, 1,3- and 1,4-butanediol di(meth)acrylate, vinyl(meth)acrylate, allyl (meth)acrylate, glycerol tri-, di- andmono(meth)-acrylate, trimethylolpropane tri-, di- andmono(meth)-acrylate, styrene, vinyltoluene, divinylbenzene,pentaerythritol tri- and tetra(meth)acrylate, di- and tripropyleneglycol di(meth)acrylate, hexanediol di(meth)acrylate, as well asmixtures thereof.

The photoinitiators preferably present in the free-radically curingsystem B) are used, for example, in quantities of 0.1 to 5 wt. %,preferably of 0.5 to 3 wt. %, relative to the total of free-radicallypolymerisable prepolymers, reactive diluents and photoinitiators. It isfavourable if the absorption thereof is within the range of wavelengthsfrom 260 to 450 nm. Examples of photoinitiators are benzoin andderivatives, acetophenone and derivatives, for example2,2-diacetoxyacetophenone, benzophenone and derivatives, thioxanthoneand derivatives, anthraquinone, 1-benzoylcyclohexanol, organophosphoruscompounds, such as for example acyl phosphine oxides. Thephotoinitiators may be used individually or in combination. Furthersynergistic components, for example tertiary amines, may moreover alsobe used.

Apart from the above-stated constituents of the systems B), these mayadditionally contain thermally activatable free-radical initiators. Fromtemperatures of 80 to 120° C., these form free radicals. Examples ofthermolabile free-radical initiators are: organic peroxides, organic azocompounds or C—C cleaving initiators, such as dialkyl peroxides,peroxycarboxylic acids, peroxydicarbonates, peroxide esters,hydroperoxides, ketone peroxides, azodinitriles or benzopinacole silylethers. Preferably used quantities are between 0.1 and 5 wt. %, relativeto the total of free-radically polymerisable prepolymers, reactivediluents and free-radical initiators. The free-radical initiators mayalso be used as a mixture.

Apart from the systems A) and B), the clear lacquers usable in theprocess according to the invention may contain transparent pigments orextenders, as well as conventional lacquer additives, such as forexample antioxidants, sensitisers, catalysts, leveling agents, dyes,rheological additives as well as light stabilisers in quantitiesconventional in lacquers.

The clear lacquer coating compositions usable in the process accordingto the invention may be formulated without solvents, in which case thesolids content thereof is 100 wt. % and is composed of the resin solidscontent formed by the systems A) plus B) as well as further non-volatileconstituents. The clear lacquer coating compositions usable in theprocess according to the invention preferably have a solids content of40 to 80 wt. %. In this case, they contain volatile organic solvents,such as for example glycol ethers, such as ethylene glycol monobutylether, diethylene glycol monobutyl ether, dipropylene glycol dimethylether, dipropylene glycol monomethyl ether, ethylene glycol dimethylether; glycol ether esters, such as ethylene glycol monoethyl etheracetate, ethylene glycol monobutyl ether acetate, 3-methoxy-n-butylacetate, diethylene glycol monobutyl ether acetate, methoxypropylacetate; esters such as butyl acetate, isobutyl acetate, amyl acetate;ketones, such as methyl ethyl ketone, methyl isobutyl ketone, diisobutylketone, cyclohexanone, isophorone; alcohols such as methanol, ethanol,propanol, butanol, aromatic hydrocarbons, such as xylene, Solves so 100(registered trademark for a mixture of aromatic hydrocarbons having aboiling range from 155 to 185° C.) and aliphatic hydrocarbons, or theycomprise aqueous coating compositions in emulsion form. The emulsifiedstate may here be achieved by addition of external emulsifiers orsystems A) and/or B) comprise systems which contain groups having aself-emulsifying action in water, for example ionic groups. The emulsionsystems may also contain organic solvents, in particular water-misciblesolvents.

Surprisingly, the clear lacquer coating compositions usable in theprocess according to the invention may be produced in a simple manner bymixing the systems A) and B) constituting the resin solids content or bymixing the constituents thereof in a solids weight ratio of 50 to 98parts of A) to 50 to 2 parts of B), preferably of 70 to 95 parts of A)to 30 to 5 parts of B) (while complying with the above-statedrequirements regarding the C═C equivalent weight of the resin solids)with the above-stated constituents which do not contribute to the resinsolids content. Mixing preferably proceeds by using commerciallyavailable and/or simply synthesised constituents, as are explainedabove. The order in which the individual constituents are added duringmixing is in principle freely selectable and is determined in accordancewith practical requirements and is only subject to the restrictionsdetermined by multicomponent systems in the case of the less preferredmulticomponent clear lacquer coating compositions. The clear lacquercoating compositions used in the process according to the invention areparticularly preferably produced by mixing a system B) and, if desired,organic solvent, into a per se complete single-component stoving clearlacquer, the resin solids content of which consists of a system A)thermally curable by addition and/or condensation reactions, whichsystem A) contains substantially no free-radically polymerisable doublebonds and substantially no groups capable of reacting in another waywith free-radically polymerisable double bonds of system B), whilecomplying with the above requirements, stated to be essential to theinvention, with regard to the composition of the resin solids contentand C═C equivalent weight of the resin solids.

In the process according to the invention, the coloured and/or effectbase lacquer layer is applied onto an optionally precoated substrate, inparticular onto precoated automotive bodies or components thereof.Examples of precoats on automotive bodies or components thereof are anelectrocoated primer layer and a sprayed surfacer layer or a two-layerprecoat consisting of an electrocoated primer layer and a secondelectrophoretically applied coating layer.

The coloured and/or effect base lacquer layer is applied by spraying acoloured and/or effect base lacquer to a dry film thickness, dependentupon the colour tone, of generally between 10 and 25 μm.

Once the coloured and/or effect base lacquer coating layer has beenapplied, there is a brief flashing-off phase, for example at 20 to 80°C., followed by application of the clear lacquer layer of the clearlacquer coating composition according to the invention to a dry filmthickness of generally between 25 and 50 μm using the wet-on-wet method.Especially when clear lacquer coating compositions containing organicsolvents and/or water are used, the coating may optionally be brieflyflashed off. It is often convenient and preferred to promoteflashing-off by heating, for example preferably with the assistance ofinfrared radiation. Application and flashing-off are preferablyperformed in such a manner that exposure to light of a wavelength ofless than 380 nm is very largely prevented.

The clear lacquer coating may, but need not necessarily, extend over theentire surface of the, for example, three dimensional substrate. It isaccordingly possible in the process according to the invention, forexample, preferably to perform coating with the clear lacquer coatingcomposition substantially only on external areas, in particular on thevisible surfaces of a three-dimensional substrate, i.e. for example notin the confined cavities of a vehicle body.

After the optional flashing-off phase, the clear lacquer coatingcomposition is exposed to high-energy radiation. Such radiationcomprises electron beam radiation or preferably ultraviolet radiation.In the case of electron beam radiation, irradiation must be performedunder an inert atmosphere; this is not necessary when using UVradiation. UV radiation sources emitting in the range of wavelengthsfrom 180 to 420 nm, preferably of 200 to 400 nm, are preferred. Examplesof such UV radiation sources are optionally doped high, medium and lowpressure mercury light sources, gas discharge tubes, such as for examplelow pressure xenon lamps, pulsed and unpulsed UV lasers, point sourcesof UV light, such as for example UV-emitting diodes, black light tubes,high energy electron flash devices, such as for example UV flash lamps.

The UV radiation sources may be of a continuously or discontinuouslyoperating design. In the latter case, laser light sources are suitable.Another possibility for providing UW sources which may be rapidly turnedon and off (pulsable sources) is to arrange mobile diaphragms in frontof the light source or UV flash lamps are used.

Light control systems conventionally used in industrial optics, such asfor example absorption filters, reflectors, lens systems, opticalfibres, may be used as ancillary devices.

The arrangement of the radiation sources is known in principle; it maybe adapted to the particular characteristics of the substrate, forexample an automotive body or the components thereof.

For example, the entirety of the substrate may be irradiated, forexample while passing through a UV irradiation tunnel or a radiationcurtain may be used which moves relative to the substrate. A pointsource of radiation or a small area light source may also be guided overthe substrate by means of an automatic apparatus.

The distance from the radiation source may be fixed or may be adapted toa certain value relative to the shape of the substrate. The radiationsources are preferably at a distance in the range from 2 to 50 cm,particularly preferably from 5 to 30 cm, away from the wet lacquersurface. Where a UV laser is used, a larger distance is possible.

The listed processing measures may, of course, also be combined. Thismay proceed in a single process stage or in temporally or spatiallyseparate process stages.

The duration of irradiation is, for example, in the range from theduration of a UV flash of for example 100 milliseconds to 5 minutes,depending upon the radiation process used and the nature and number ofthe UV radiation sources. A duration of irradiation, i.e. the period forwhich the clear lacquer coating layer is actually exposed to UVradiation, is preferably less than 5 minutes.

When coating three-dimensional substrates of a complicated geometry,such as for example automotive bodies, with radiation-curing lacquers,the problem frequently arises of inadequate curing due to insufficientaccessibility for the high-energy radiation, for example in shadedareas, such as for example cavities, folds and other structurallydetermined undercuts. This problem may be avoided in the processaccording to the invention, as the degree of curing of the clear lacquerachievable with the process according to the invention is entirelysufficient for the shaded areas, i.e. the degree of curing of the clearlacquer achievable by the thermal curing stage is sufficiently high inthose areas which are not visible to the observer. Moreover, it ispreferred in the process according to the invention to provide a coatingwith the clear lacquer coating composition substantially only onexternal areas, in particular on visible surfaces of a three-dimensionalsubstrate, i.e. for example not in the confined cavities of a vehiclebody. In this case, the issue of possibly inadequate curing of the clearlacquer layer in shaded areas is virtually irrelevant. If desired,shaded areas may, however, be irradiated, for example by using point,small area or omnidirectional radiation sources, optionally with anautomatic moving apparatus, for example for irradiating internalcavities or edges. Once irradiated, the clear-lacquer coating layer istack-free and the substrate is sent to the stoving process, during whichthe clear lacquer coating layer, together with the coloured and/oreffect base lacquer layer, is stoved at elevated temperatures of forexample 80 to 160° C. or cured at lower temperatures of for example 20to 60° C. The process according to the invention provides the advantageof making it possible, if desired, to shorten the conventional stovingperiod, in particular in comparison with stoving period required for acorresponding clear lacquer coating layer produced from a clear lacquercoating composition not containing system B). Final hardness is alsoachieved more rapidly in the process according to the invention, forexample immediately after the stoving process.

The process according to the invention performed using the knownwet-on-wet principle allows the production of base lacquer/clear lacquertwo-layer lacquer coatings, in particular on motor vehicles and thecomponents thereof, having outstanding optical/aesthetic qualities. Theprocess according to the invention may be used not only for automotiveoriginal lacquer coating and component lacquer coating but also forautomotive repair lacquer coating. Running of the clear lacquer downvertical surfaces during stoving is effectively prevented. The clearlacquer layers obtained using the process according to the inventionexhibit reduced susceptibility to chemicals and scratching, inparticular to acid and washing scratches. Surprisingly, the above-statedadvantages may be achieved with the process according to the invention,although the resin solids content of the clear lacquer coatingcomposition used in the process according to the invention is of asimple composition and the crosslinking systems A) and B) forming theresin solids content do not crosslink together. The clear lacquercoating compositions may be formulated from known, for examplecommercially available components. Production of multilayer lacquercoatings

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1 (COMPARISON)

A silver-coloured aqueous base lacquer is applied by spraying to a dryfilm thickness of 15 μm onto a metal sheet provided with a cataphoreticprimer and surfacer layer and is predried for 10 minutes at 80° C. Acommercially available automotive original clear lacquer (52 wt. % resinsolids content, formed from, hydroxy-functional acrylate binder andmelamine resin crosslinking agent) is then applied by spraying to a dryfilm thickness of 40 μm, is flashed off for 10 minutes at roomtemperature then for 1 minute promoted by infrared radiation and is thenstoved for 15 minutes at 140° C. (object temperature).

EXAMPLE 2

1000 g of the clear lacquer from Example 1 are homogeneously mixed with52 g of a mixture prepared from 72 parts by weight of a hexafunctional,aliphatic urethane acrylate (theoretical molecular weight 800), 22 partsby weight of hexanediol diacrylate, 1.5 parts by weight of an acylphosphine oxide type photoinitiator and 1.5 parts of an acetophenonetype photoinitiator.

The resultant clear lacquer is then processed as in Example 1, exceptthat, between flashing-off and stoving, the clear lacquer surface isexposed to the light from a UV flash 10 times within 40 seconds(xenon-filled UV flash lamp, 3500 Ws, object distance 20 cm).

EXAMPLE 3

1000 g of the clear lacquer from Example 1 are homogeneously mixed with104 g of the mixture stated in Example 2.

The resultant clear lacquer is then processed as in Example 2.

EXAMPLE 4

1000 g of the clear lacquer from Example 1 are homogeneously mixed with234 g of a mixture prepared from 75 parts by weight of a hexafunctional,aliphatic urethane acrylate (theoretical molecular weight 800), 22 partsby weight of tripropylene glycol diacrylate, 1.5 parts by weight of anacyl phosphine oxide type photoinitiator and 1.5 parts of anacetophenone type photoinitiator.

The resultant clear lacquer is then processed as in Example 2.

EXAMPLE 5

1000 g of the clear lacquer from Example 1 are homogeneously mixed with26 g of a mixture prepared from 75 parts by weight of a hexafunctional,aliphatic urethane acrylate (theoretical molecular weight 1000), 22parts by weight of trimethylolpropane triacrylate, 1.5 parts by weightof an acyl phosphine oxide type photoinitiator and 1.5 parts of anacetophenone type photoinitiator.

The resultant clear lacquer is then processed as in Example 2.

EXAMPLE 6

1000 g of the clear lacquer from Example 1 are homogeneously mixed with88 g of the mixture stated in Example 5.

The resultant clear lacquer is then processed as in Example 2.

EXAMPLE 7

1000 g of the clear lacquer from Example 1 are homogeneously mixed with78 g of a mixture prepared from 75 parts by weight of a hexafunctional,aliphatic urethane acrylate (theoretical molecular weight 800), 22 partsby weight of trimethylolpropane triacrylate, 1.5 parts by weight of anacyl phosphine oxide type photoinitiator and 1.5 parts of anacetophenone type photoinitiator.

The resultant clear lacquer is then processed as in Example 1, exceptthat, between flashing-off and stoving, the clear lacquer surface issubjected to UV irradiation (medium pressure mercury light source, 120W/cm, belt speed 3 m/min, object distance 10 cm).

Table 1 shows the properties of the multilayer lacquer coatings obtainedin Comparative Example 1 and in Examples 2 to 7 according to theinvention.

TABLE 1 Pendulum Pendulum hardness after Scratch hardness xylene test¹⁾resistance²⁾ Acid Example (oscillations) a b resistance³⁾ 1 91 27 90 5511 2 97 42 93 62 20 3 103 105 89 75 22 4 115 117 90 71 ./. 5 95 90 93 6022 6 108 98 93 63 24 7 120 120 92 79 >30 

¹⁾The xylene test is performed by applying a cotton wool swab saturatedwith xylene onto the multilayer coating for 10 minutes. Pendulumhardness is measured in each case after a recovery time of 1 hour.

²⁾The values stated are initial gloss before exposure to scratching(column a) and residual gloss after the exposure to scratching describedbelow (column b), in each case determined by gloss measurement at anangle of 20°. Exposure to scratching is performed using thereciprocating stroke method with model no. 265 Erichsen-Peters block.Dimensions are: 75×75×50 mm, area of base =3750 mm². Weight is 2 kg. Apiece of 2.5 mm thick woolen felt of dimensions 30×50 mm is stuck underthe sanding block with adhesive tape. 1 g of a water-soluble abrasivepaste is then uniformly distributed over the application surface. 10to-and-fro strokes are performed in 9 seconds. The reciprocating strokeis made parallel to the 75 mm edge of the block, with the distancecovered being 90 mm in one direction. The surface is then rinsed withcold water and the gloss measurement performed.

³⁾Acid resistance is tested using the 10% sulfuric acid droplet test.The test sheets are placed on a hotplate and heated to 60° C. Care mustbe taken to ensure that the sheets lie flat on the hotplate duringheating. At the end of the heating phase, i.e. at 60° C, one droplet ofthe acid is applied onto the clear lacquer surface each minute. Thetotal time is 30 minutes. On completion of the test period, the lacquercoating is washed with water. The value stated is the period of exposurein minutes at which the first visible film change (swelling) occurred.

What is claimed is:
 1. A process for coating a substrate with a baselacquer layer and a clear topcoat lacquer layer, comprising: applying abase lacquer layer onto a substrate, applying an uncured clear topcoatlacquer layer onto the base lacquer layer in a wet-on-wet manner,exposing the uncured clear topcoat lacquer layer to high-energyradiation to partially cure the clear topcoat lacquer layer, andthereafter jointly stoving the base lacquer layer and the clear topcoatlacquer layer, wherein the base lacquer layer comprises at least onefeature selected from the group consisting of a color and an effect,wherein the clear topcoat lacquer layer has a resin solids contentincluding 50 wt. % to 98 wt. % of a thermally curable polymer system (A)which is thermally curable by addition reactions, condensationreactions, or both, the system (A) containing substantially nofree-radically polymerizable double bonds and substantially no groupscapable of reacting in another way with free-radically polymerizabledouble bonds of system (B), and 2 wt. % to 50 wt. % of a free-radicalcuring polymer system (B) which is curable via high-energy radiationmediated free-radical polymerization of olefinic double bonds, whereintotal weight percentages add up to 100 wt. %, and wherein C═C equivalentweight of the resin solids total content from system (A) and system (B)is between 300 and 10,000.
 2. The process of claim 1 wherein the cleartopcoat lacquer layer has a resin solids content including 70 wt. % to95 wt. % of the thermally curable polymer system (A) and 5 wt. % to 30wt. % of the free-radical curing polymer system (B).
 3. The process ofclaim 1 wherein the C═C equivalent weight of the entire resin solidscontent formed from the thermally curable polymer system (A) and thefree-radical curing polymer system (B) is between 400 and
 1500. 4. Theprocess of claim 1 wherein the substrate is a motor vehicle body.
 5. Aclear lacquer coating composition for coating a substrate, comprising aresin solids content including: 50 wt. % to 98 wt. % of a thermallycurable polymer system (A) which is thermally curable by additionreactions, condensation reactions, or both, the system (A) containingsubstantially no free-radically polymerizable double bonds andsubstantially no groups capable of reacting in another way withfree-radically polymerizable double bonds of system (B), and 2 wt. % to50 wt. % of a free-radical curing polymer system (B) which is curablevia high-energy radiation mediated free-radical polymerization ofolefinic double bonds, wherein total weight percentages add up to 100wt. %, and wherein C═C equivalent weight of the resin solids totalcontent from system (A) and system (B) is between 300 and 10,000.
 6. Theclear lacquer coating composition of claim 5 wherein the composition hasa resin solids content including 70 wt. % to 95 wt. % of the thermallycurable polymer system (A) and 5 wt. % to 30 wt. % of the free-radicalcuring polymer system (B).
 7. The clear lacquer coating composition ofclaim 5 wherein the C═C equivalent weight of the entire resin solidscontent formed from the thermally curable polymer system (A) and thefree-radical curing polymer system (B) is between 400 and
 1500. 8. Theclear lacquer coating composition of claim 5 wherein the substrate is amotor vehicle body.